1. Field of the Invention
The invention is in the field of recovery of values from scrap solder and solder skimmings.
2. Description of the Prior Art
Solder baths used in can manufacturing are covered with a layer of ammonium chloride flux to prevent oxidation of the solder. Zinc chloride is added to the flux to reduce fuming of the molten ammonium chloride. Lead oxide present reacts with the chlorides to form lead chloride. In skimming off the slag which is formed as the cans are soldered some of the flux and solder is removed because of the inefficiency of the skimming procedure. The skimmings after they harden contain solder chunks and granular lead chloride in a matrix of solid ammonium chloride and zinc chloride. These skimmings are referred to herein as solder skimmings or solder bath skimmings.
The existing process for recovering solder from solder bath skimmings consists of melting the skimmings in a gas-fired furnace, tapping the molten solder and discarding the top layer of zinc chloride, ammonium chloride, and lead chloride. In the process of furnacing, an ammonium chloride fume is emitted from the stack at a level far in excess of current air pollution control limits. Air pollution correction equipment for the off-gas is costly. High energy wet venturi scrubbers will not meet present air pollution codes on plume opacity. Electrostatic precipitators are prohibitively expensive since the elements must resist corrosive attack by the acidic fumes.
In view of the disadvantages of prior processes for recovering solder from scrap solder skimmings, there is a need for a method for economically and practically recovering solder from the solder skimmings which is within acceptable air pollution control limitations.